Update: since posting this bug here, others have reported also seeing this behavior with voices other than Alex.
Original Report: This bug appears to be dependent upon a very specific use case and has an effective workaround.
If you are using Alex as your default TTS voice, you may find that VoiceOver will switch to use another voice whenever it needs to announce punctuation.
From testing, it appears that this behavior is likely to present itself if you have only the Alex TTS voice enabled. It is also likely that your region setting may be a factor, as this problem has been most consistent for one of our Team who has this set to the UK.
A quick and simple test to see if you are likely to encounter this behavior is to go to a text field on your device so that the onscreen keyboard appears. Now switch to the punctuation keyboard. If VoiceOver continues using Alex when you touch any of the punctuation keys, then it’s probably safe to assume that you will not see this problem.